Method and controller for the adaptive control of at least one component of a technical plant

ABSTRACT

The invention relates to a method in which at least one component of a technical plant is controlled by means of a PI controller. The actual value of the regulating parameter is continuously determined during operation of the plant and the amplification factor of the PI controller is altered depending on the time relationship of the actual value, until the actual value remains with a tolerance band relative to the set value. The invention further relates to a controller for carrying out said method.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/DE03/00793, filed Mar. 12, 2003 and claims the benefit thereof.The International Application claims the benefits of German Patentapplication No. 10213533.9 DE filed Mar. 26, 2002, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for controlling at least one componentof a technical plant, and a corresponding controller.

BACKGROUND OF THE INVENTION

The starting point is a conventional control concept according to whicha control variable of a component of the technical plant is held asclosely as possible to the set value of the control variable by means ofa PI controller. To do so, the PI controller determines a correctionaction for an actuator assigned to one of the components from thedifference between the set value and actual value of the controlvariable (control deviation), so that the control variable reaches therequired set value as quickly and accurately as possible and at the sametime moves as little as possible from this set value in the course ofthe control action. A strong oscillation of the actual value of thecontrol variable should particularly be avoided so that the controlsystem consisting of the components of the technical plant and thecontroller does not have a tendency to hunt.

For the control of power station components it is, for example, knownfor the control parameters of a PI controller used for control,particularly the control ratio and integral-action time, to be set to aconstant value in advance in each case and not to be altered againduring the control action when operating the plant. The advantage ofthis is that only a few parameters, particularly the aforementionedcontroller parameters, have to be set when using a PI controller, andvery often no further adjustments have to be made during the controlaction.

However, the use of parameter values set in advance for the PIcontroller is not always the optimum solution, particularly for thecontrol of power station plants, because in the course of time duringthe operation of the power station plant the dynamic behavior of thetechnical process effected by the components of the technical plant canchange. Thus, after a change of this kind in the dynamic behavior of thecomponents the controller parameters set in advance are no longeroptimum for the new operating conditions now present and under certaincircumstances can even lead to failure of the control system, so thatthe desired control effect can no longer be achieved. For example, aftera change in the dynamic behavior of the controlled system (components)and thus the control circuit formed by the components and the PIcontroller, the actual value of the control variable can have a tendencytowards unwanted oscillations during the control action that can lead toinstability of the controlled system. To make a control system of thiskind again suitable for use during the operation of the technical plant,it is usually necessary to at least take the affected components of thetechnical plant out of service, to re-adjust the controller parameterscorresponding to the changed dynamic behavior of the controlled system,which possibility renders comprehensive test runs necessary beforerenewed commissioning, and then to restore the components, includingtheir controller, to service.

Apart from the fact that a controlled circuit of a technical plant thathas become functionally unserviceable can be hazardous to operators andpotentially damaging to machines, particularly if a control system thathas become unstable creates corrections that are too great and/or actualvalues of the control variable that are far too large, and because ofthe downtime required to repair the control system a production failureof the technical plant is practically unavoidable. If the technicalplant is a power station, the failure of a component of the technicalplant can, for example, jeopardize the supply of electrical power to anarea.

The use of constant values for the PI controller leads to problems,particularly where used for control in power stations with a steamprocess, because in power stations of this kind the control systems tobe controlled often have a non-linear behavior. A set of parametervalues for the PI controller that can be regarded as optimum for anoperating state of the controlled components can under a differentoperating condition of these components produce control results thatbecause of the existing non-linearities are at least of only limiteduse. Thus if there is an unavoidable shift in the working point of anon-linear components to be controlled or the technical plant duringoperation of the technical plant, the preset controller parameters arepossibly no longer suitable for the new operation point (working point)for producing the desired control effect. This can even have the effectthat a PI controller has an optimum control behavior for an operatingpoint of the component, but results in completely inadequate or evendangerous control outcomes when a changeover to a new operating pointoccurs.

SUMMARY OF THE INVENTION

The object of the invention is therefore to specify an improved methodand a controller for controlling at least one component of a technicalplant. This should in particular overcome the described disadvantagesfrom prior art. Furthermore, a control method in accordance with theinvention and a controller in accordance with the invention shouldreduce the cost of setting the controller parameters. Furthermore, amethod in accordance with the invention and a controller in accordancewith the invention should enable a particularly simple start-up of acomponent to be controlled.

With regard to the method, the object is achieved in accordance with theinvention by a method for controlling at least one component of atechnical plant by means of a PI controller that has a control ratio andan integral-action time as controller parameters, with the followingsteps.

The integral-action time is preset.

An initial value of the control ratio is preset.

At least one set value of a control variable of the component is presetand during the operation of the technical plant, the actual value of thecontrol variable is constantly determined and the control ratio ischanged relative to the time response of the actual value, until theactual value of the control variable remains within a tolerance bandwith regard to the set value.

The invention is based on the consideration that the controllerparameters can then be particularly well specified if the behavior ofthe controlled system, i.e. the interaction between the PI controllerand the component to be controlled, are observed during the operation ofthe technical plant and the control variable of the control ratio of thePI controller is automatically changed during the operation of thetechnical plant corresponding to the behavior of the actual value, untilthe actual value of the control variable is within the stated tolerancebend with regard to the defined set value and remains there. Changes tothe control ratio therefore do not take place continuously but onlyuntil the actual value of the control variable exhibits a requiredbehavior with regard to the tolerance band, i.e. its value moves advalorem within the tolerance band during the operation of the technicalplant. If in the course of the operation of the controlled component achange in its dynamic behavior now takes place, caused for example bymaterial wear and/or deposits of operating or auxiliary materials of thecomponent, or by aging of parts of the component, the control ratio whencarrying out the method in accordance with the invention is not changedagain until the actual value of the control variant moves ad valoremfrom the tolerance band. This new ongoing change in the control ratiocontinues only until the actual value again enters the tolerance bandand remains there.

By means of the method in accordance with the invention, a method forthe automatic adaptation of the control ratio of the PI controller isrealized, whereby during the operation of the technical plant thecontrol ratio continues to be automatically changed relative to thebehavior of the actual value of the control variable until it leads to adesired control behavior of the control component. A further change inthe control ratio takes place only if the actual value of the controlvariable again moves ad valorem from the tolerance band. The ongoingalteration of the control ratio can take place by a fixed amount, forexample by means of a stepwise change in the current value of thecontrol ratio.

With the method in accordance with the invention, the integral-actiontime control parameter is specified in advance and thus is notcontinuously altered during the operation of the technical plant.

Investigations of controlled systems have shown that setting theintegral-action time control parameter in advance is usually adequate toachieve a good, desired control behavior of the controlled system. It isthus not necessary to also continuously change the integral-action timecontrol parameter in order to achieve a good control result during theoperation of the technical plant. The advantage of this is mainly thatthe controlled system dynamics, e.g. for further investigations or testpurposes, are easier to describe and model, because due to theintegral-action time being specified in advance, the dynamic behavior ofthe PI controller does not change and a suitable, for examplemathematical, description of the controlled system can easily be givenand used.

Advantageously, the integral-action time is determined from system timeconstants, particularly from the sum of the system time constants of thecomponent to be controlled.

The system time constants of a system to be controlled influence thespeed with which the system reacts to a change of at least one of itsinput signals by changing at least one of its output signals. Almost anypractical control system is subject to a delay, as it is called, andthus also includes at least one system time constant in itscorresponding mathematical model equation. For example a controlledsystem that can be described by a third order mathematical orderequation includes three system time constants. Knowledge of the valuesof the system time constants for a controlled system under considerationenables the delay of the controlled system to be well estimated.

It is particularly advantageous if the sum of the system time constantsof the system to be controlled is formed and the integral-action time isdetermined by means of this sum and, in particular, is set as a multipleof this sum. Advantageously, this multiple varies in the 0.1 to 2.5range.

In many practical cases that occur, tests have shown that a multiplewith a value of approximately 1.5 produces good results, i.e. that theintegral-action time of the PI controller in such cases is to be set toa value corresponding to one and half times the sum of the system timeconstants. Furthermore, multiples with a value of approximately 0.7 alsoproduce good results, i.e. the integral-action time of PI controller insuch cases is to be advantageously set to a value corresponding to 70%of the sum of the system time constants. When doing so, however, it isto be noted that the effect of the amount of integral action of the PIcontroller reduces with increasing value of the named multiple.

In a further advantageous embodiment of the invention, the control ratiois reduced in step 4 in the method in accordance with the invention ifthe time pattern of the actual value has a dwell time during which theactual value shows a value within the tolerance band that is smallerthan a first defined time period.

With this form of embodiment, oscillations in the time response of theactual value in particular should be detected, in particular displayingthemselves in that the actual value assumes a value within the toleranceband during the control action once again and subsequently enters andleaves the tolerance band. The dwell time is now determined during whichthe oscillating actual value moves within the tolerance band before itleaves it once again. The shorter this dwell time the faster the actualvalue changes relative to time, so that in a case of this kind thecontrol ratio of the PI controller is to be reduced in order to leavethe actual value within the tolerance band during the operation of thetechnical plant. If the dwell time is less than the first defined timeperiod, an excessive oscillation of the actual value (the actual valuesgoes “too fast” through the tolerance band) is assumed and the controlratio is reduced thus preventing an intolerable oscillation of this kindof the actual value beyond the limits of the tolerance band.

The first defined time periods can, for example, be determined relativeto the values of the system time constants, particularly relative to thevalue of the sum of the system time constants. Thus, with theaforementioned comparison, the dwell time of the actual value is set ina technically useful relation to the delay of the controlled system(described by the system time constants), because an absolute definitionof this, which is an excessive oscillation, cannot be given.

For example, a temperature change within a few minutes can indicate afast process whereas a pressure change within a few minutes is usuallymore likely to be characteristic of a slow process.

Advantageously the control ratio in step 4 is then only reduced if, inaddition, a first change rate of the actual value is greater than asecond change rate of the set value.

This embodiment of the invention takes account of the fact that the setvalue can not only be a constant variable but can also be a changeable,particularly a fluctuating and/or oscillating variable. Values of theset value that change in this manner during the control action couldlead to fluctuations and/or oscillations of the actual value of thecontrol quantity, similar to an incorrectly set value of the controlratio. However, such fluctuations and/or oscillations of the actualvalue with the corresponding set value also fluctuating and/oroscillating are even desirable because the actual value of the controlvariable should follow the set value of the control variable as closelyas possible during the control action.

In order in such cases to prevent an (unwanted) reduction in the controlratio due to a fluctuating actual value, it is provided with this formof embodiment that a first change rate of the actual value and a secondchange rate of the set value be determined in order to then establish,from a comparison of these two rates, whether the control ratio is to bereduced or not.

If the actual value changes more quickly than the set value (the firstchange rate is in this case greater than the second change rate), thecontrol ratio is to be reduced because in this case the fluctuations ofthe actual value are due not only to a fluctuating set value, but alsodue to a set control ratio that is ad valorem too great.

In a different case, if the value of the actual value changes lessquickly than the value of the set value, the control ratio is notreduced because from this it is assumed that the fluctuations of theactual value in this case are due mainly to fluctuations in the setvalue and consequently a reduction in the control ratio is not necessaryand would also not be useful for success of the control system.

In a further advantageous embodiment of the invention, the control ratiois increased in step 4 of the method in accordance with the invention ifthe time characteristic of the actual value has a rise time thatincludes the time period from the start of a change of the set value upto reaching a momentary value of the actual value within the toleranceband, that is greater than a second, defined time period.

With this form of embodiment, the control ratio is increased if thereaction of the controlled system to a change in the set value is tooslow. In addition, the aforementioned rise time of the actual value isdetermined corresponding to the time period between a change in the setvalue and an initial attainment by the actual value of a value withinthe tolerance band. The rise time is thus a measure of how fast theactual value of the controlled system reacts to a change, particularly asudden change, in the set value, and reaches a (desired) value withinthe tolerance band relative to the set value. The second defined timeperiod serves as a criterion in the event that the rise time of theactual value is seen to be too great and the reaction time of thecontrolled system to a change in the set value thus as too slow.Advantageously, this second defined time period is determined from thesystem time constants of the component to be controlled, particularlyfrom the sum of the system time constants. The rise time is thus setrelative to the delay in the control system so that a definition that isto be seen as a reaction to the control system that is too slow can beset in a useful technical relationship to the delay of the controlledsystem.

The invention furthermore leads to a controller for controlling at leastone component of a technical plant, designed as a PI controller andincludes a control ratio and an integral-action time as controllerparameters, whereby the controller has at least the following controllercomponents.

A first controller input by means of which the controller can besupplied with a defined value for the control ratio.

A second controller input by means of which the controller can besupplied with a control ratio.

A third controller input by means of which the controller can besupplied with a control variable of the component, and an adaption unitby means of which the actual value of the control variable can beconstantly determined during the operation of the technical plant andthe control ratio can be changed relative to the time relationship ofthe actual value, until the actual value of the control variable remainswithin a tolerance band relative to the set value.

The integral-action time is advantageously determined from the systemtime constants, particularly from the sum of the system time constants,of the controlled component.

In an advantageous embodiment, the control ratio is reduced by means ofthe adaption unit if the time relationship of the actual value has adwell time during which the actual value assumes a value within thetolerance band that is less than a first defined time period.

The control ratio is then advantageously reduced by the adaption unitonly if in addition a first change rate of the actual value is greaterthan a second change rate of the set value.

In a further advantageous embodiment of the invention, the control ratiois increased by means of the adaption unit if the time relationship ofthe actual value is a rise time that covers the time period from thestart of a change in the set value up to reaching an instantaneous valueof the actual value within the tolerance band that is greater than asecond defined time period.

The explanations and notes given in conjunction with a representation ofthe method in accordance with the invention, and the advantages shown,can be applied as appropriate to a controller in accordance with theinvention and to its forms of embodiment and are therefore not repeatedat this point.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary of an embodiment of the invention is explained in moredetail below, where;

FIG. 1 shows a graphic representation of an example of the behavior ofthe actual value, with a dwell time and a first defined time periodbeing given as parameters for use in accordance with the invention;

FIG. 2 shows an example of the behavior of the set and actual valueswith both quantities having an oscillation and parameters fordetermining the rate of change of the set and actual value being usedfor use in accordance with the invention;

FIG. 3 shows a further example of the time response of the actual value,with a rise time and a further defined time period being given asparameters for use in accordance with the invention; and

FIG. 4 shows a controller in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 show an example of the time response of an actual value I, thathas an oscillation and even over a long time period does notsatisfactorily approximate to a defined set value S, and thus inparticular does not move within a tolerance band Tb.

The illustration in FIG. 1 shows the time behavior of the set and actualvalues of a controlled component of a technical plant. At time pointt=50 (for example t=50 sec.), the set value S changes suddenly from S=0to S=1 and remains constant from time point t=50. With a controlledsystem, it is desirable for the actual value I of the associated controlvariable to follow the time response of the set value S as closely aspossible, whereby on one hand the set value S should be reached asquickly as possible and on the other hand excessive over- or undershootsof the actual value I relative to the set value S must be avoided inorder to obtain a stable controlled system. In particular, oscillationsof the actual value I that do not decay over time and/or the amplitudeof which has values that do not lie within the tolerance band Tb, oreven considerably overshoot it, are particularly to be avoided. Thetolerance band Tb is to be matched to a particular concrete area of useand should reflect a permissible deviation of the actual value I fromthe desired set value S during the control action.

The limits of the tolerance band Tb need not in this case be symmetricalrelative to the set value S. Rather they can be matched to therequirements of a specific application.

In the example in FIG. 1 it can be seen from the time response of theactual value I that the control ratio Kp of the basic controlledcomponent would be too great, i.e. the PI controller used would reactwith an excessive proportional gain to a control deviation determined bythe difference between the set and actual values, which in the caseillustrated in FIG. 1 would lead to an undesirable oscillation of theactual value I.

To obtain a stable control, particularly to be able to meet therequirements of the time response of the actual value with regard to thetolerance band Tb, it is necessary, and provided by the invention, tocontinuously change the control ratio Kp until the actual value I of thecontrol quantity remains within the tolerance band Tb. In this examplethe control ratio Kp is to be reduced. This is shown particularly by thefact that a dwell time T11 is less than a first defined time period T1.This can be approximately interpreted as the actual value I passing “toofast” through the tolerance band Tb during its time response, whichindicates an undesirable oscillation of the actual value I. Thecomparison period (first defined time period T1) is in this caseadvantageously determined by means of the system time constants of thecomponent to be controlled, so that the fact that the aforementionedterm “too fast” should be defined relative to the system-dependent delayof the controlled component can be taken into account.

The continuous reduction of the control ratio Kp is ended as soon as thetime response of the actual value moves to within the tolerance band Tband remains there. Alternatively, or additionally, the reduction in thecontrol ratio Kp can be achieved by multiplication with a constant valuebetween 0 and 1, and can also be achieved each time the actual value ispassed through the tolerance band.

FIG. 2 shows the time response of the set value S and the actual valueI, with the set value S having an oscillation, i.e. no areas of constantbehavior particularly in contrast to FIG. 1.

As a consequence, the actual value I that should follow the timeresponse of the set value S as closely as possible has an oscillatingpattern.

The conclusion that the control ratio Kp of the PI controller used isset too high, therefore causing the oscillation of the actual value Ineed not necessarily be drawn from the example of the oscillation of theactual value I. The lowering of the control ratio Kp in a case such asthis could lead to completely unsatisfactory control action.

It is much more a matter of checking whether the instantaneous set valueof the control ratio Kp is responsible for the oscillation of the actualvalue I or whether the oscillation of the actual value I is causedmerely by the desired following by the actual value I relative to theoscillating set value S in the example in FIG. 2.

The control ratio Kp should then only be reduced if the actual value Ichanges more quickly than the set value S.

To determine the named change rates of set value S and actual value I,FIG. 2 shows an example for a time period Δt of the change ΔS in the setvalue S during the reference time period Δt and the change ΔI of theactual value I. The quotient ΔS and Δt or ΔI and Δt enables the namedrates of change of the actual value I and set value S to be determined.

In the example in FIG. 2, the rate of change of the actual value I(first rate of change) is less than the rate of change of the set valueS (second rate of change). It can therefore be concluded that theoscillations of the actual value I are due to the oscillations of theset value S, so that in this case the control ratio Kp of the PIcontroller used is not to be reduced.

FIG. 3 shows an example of the time response of the actual value I,whereby it only reaches the defined set value S relatively slowly.

With a time response of this kind of the actual value I, it can beconcluded that the control ratio Kp of the PI controller used is set toolow, i.e. the PI controller reacts to the difference between the set andactual values (control deviation) with a proportional gain that is toosmall.

To determine whether the time response of the actual value I follows theset value S too slowly and the control ratio Kp therefore is to beraised, parameters, i.e. a rise time T22 and a second defined timeperiod T2, are shown in the example in FIG. 3.

The rise time T22 in this case includes the time period from the startof a change of the set value S to achievement of an instantaneous valueof the actual value I within the tolerance band. If this rise time T22is greater than the second defined time period T2, the componentcontrolled by a PI controller then reacts too slowly to a set valuechange and the control ratio Kp is to be increased.

Advantageously, the second defined time period T2 is determined from thesystem time constants of the component to be controlled, so that,depending on the delay of the controlled component (controlled system),it can be correctly, technically determined whether the rise time T22 ofthe actual value I is too great and the control ratio Kp is therefore tobe increased.

The continuous increase in the control ratio Kp in the case in FIG. 3continues until the actual value I remains within the tolerance band Tb.

FIG. 4 shows a controller R in accordance with the invention.

The controller R is used to control at least one component of atechnical system and is designed as a PI controller.

The controller R has a control ratio Kp and an integral-action time Tnas control parameters.

A defined integral-action time Tn can be applied to a first controllerinput E1.

The control ratio Kp can be applied to a second control input E2 and athird control input E3 receives a set value S of a control quantity ofthe component.

The controller R also has an adaption unit A by means of which theactual value I of the control variable can be determined during theoperation of the technical plant and the control ratio Kp can be changedrelative to the time response of the actual value I, until the actualvalue I of the control variable remains within a tolerance band Tbrelative to the set value S.

At the start of the control action, an initial value Kp0 for the controlratio Kp is applied to input E2. This initial value Kp0 is then changedconstantly during the control action relative to the time response ofthe actual value, until the actual value I of the control variableremains within a tolerance band Tb relative to the set value S. Thecontroller R also has a controller output Y that delivers a correctingvariable by means of which the component to be controlled is controlledin order to achieve a required behavior of the actual value I.

It is pointed out that the terms control input (E1, E2, E3) are not tobe limited in design to the fact that a physical connection to which thenamed quantities are to be applied must be present with a controller inaccordance with the invention.

The term controller input should rather comprise all methods by means ofwhich the PI controller can be supplied with the named variables.

In the case of an analogous realization of the PI controller, forexample by means of connected operational amplifiers, the termcontroller input should comprise all connection possibilities of theoperational amplifier by means of electronic components, which in theirinterconnection realize a defined value for at least one of thevariables Tn and Kp.

In the case of digital realization of the PI controller, for example adigital processor, the term controller input comprise include all memoryareas in which values for the controller parameters Tn and Kp aredescribed and/or from which these values can be exported.

1. A method for controlling a component of a technical plant by aproportional-plus-integral (PI) controller that has control parametersincluding a control ratio indicative of a proportional gain and anintegral-action time, comprising; defining a value of theintegral-action time; defining an initial value of the control ratio;defining a set value of a control quantity of the component; inputtinginto the controller the value of the integral-action time, the initialvalue of the control ratio and the set value of the control quantity;determining the actual value of a controlled variable during operationof the technical plant; adapting the value of the control ratio relativeto a time response of the actual value until the actual value of thecontrolled variable remains within a tolerance band relative to the setvalue during operation of the technical plant; and reducing the value ofthe control ratio if the time response of the actual value has a dwelltime during which the actual value of the controlled variable varieswithin the tolerance band, said dwell time being smaller than a firstdefined time period during operation of the technical plant, wherein aduration of the dwell time relative to a duration of the first definedtime period is selected to determine a sufficiently fast rate of changeof the controlled variable relative to a time constant of the component,and wherein the reduction of the value of the control ratio isconfigured to reduce the rate of change of the controlled variable. 2.The method in accordance with claim 1, wherein the integral-action timeis determined from the system time constants.
 3. The method inaccordance with claim 1, wherein the integral-action time is determinedfrom the sum of the system time constants of the component to becontrolled.
 4. The method in accordance with claim 1, wherein thecontrol ratio is reduced if a first change rate of the actual value isgreater than a second change rate of the set value.
 5. The method inaccordance with claim 1, wherein the control ratio is increased if thetime response of the actual value has a rise time that includes theperiod from the start of a change of the set value until reaching aninstantaneous value of the actual value within the tolerance band thatis greater than a second defined time period.
 6. Aproportional-plus-integral (PI) controller for controlling a componentof a technical plant, comprising: a logic element having a control ratioindicative of a proportional gain and an integral-action time; a firstcontroller input adapted to provide the controller with a defined valuefor the integral-action time; a second controller input adapted toprovide the controller with the control ratio; a third controller inputadapted to provide the controller with a set value of a control quantityof the component; and an adaption device that applies the actual valueof a controlled variable during the operation of the technical plant sothat the value of the control ratio can be adapted relative to the timeresponse of the actual value until the actual value of the controlledvariable remains within a tolerance band relative to the set valuewherein the value of the control ratio is reduced by the adaptationdevice if the time response of the actual value has a dwell time duringwhich the actual value of the controlled variable within the toleranceband, said dwell time being smaller than a first defined time period,wherein a duration of the dwell time relative to a duration of the firstdefined time period is selected to determine a sufficiently fast rate ofchange of the controlled variable relative to a time constant of thecomponent, and wherein the reduction of the value of the control ratioby the adaptation device is configured to reduce the rate of change ofthe controlled variable.
 7. The controller in accordance with claim 6,wherein the integral-action time is determined from system timeconstants.
 8. The controller in accordance with claim 6, wherein theintegral-action time is determined from the sum of the system timeconstants of the component to be controlled.
 9. The controller inaccordance with claim 6, wherein the control ratio is reduced by theadaptation unit if additionally a first change rate of the actual valueis greater than a second change rate of the set value.
 10. Thecontroller in accordance with claim 6, wherein the control ratio isincreased by the adaptation unit if the time response of the actualvalue has a rise time that includes the time period from the start of achange of the set value until achievement of an instantaneous value ofthe actual value within the tolerance band, that is greater than asecond defined time period.